I'm not comfortable making any final conclusions yet (and have a second copy of this lens arriving today), but the new lens is looking very good so far.

The version II lens is overall, sharper wide open than the version I - you can compare the Canon EF 70-200mm f/2.8 L IS I and II USM Lenses using the mouseover feature in the ISO 12233 chart tool. Increase the focal length for both lenses to see how this improved image quality carries over to extender results. CA (Chromatic Abberation) is also improved - noticeable in the corners (bottom crop) at 200mm.

The 70-200mm f/2.8 L IS II Lens focuses very fast - even faster than the version I lens. IS is quiet and well-behaved (very little jumping image in viewfinder). Build quality is excellent - though somewhat large, the lens feels great in use.